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FABRICATION OF 316L STAINLESS STEEL (SS316L) FOAM VIA POWDER COMPACTION METHOD ZULAIKHA BTE ABDULLAH A thesis as a partial fulfillment of the requirements for the award of the Master of Mechanical Engineering Faculty of Mechanical and Manufacturing Engineering Universiti Tun Hussein Onn Malaysia JUNE 2015

FABRICATION OF 316L STAINLESS STEEL (SS316L) FOAM … · vi ABSTRAK Logam berbusa adalah struktur sel yang diperbuat daripada logam dan mengandungi liang-liang di dalam strukturnya

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FABRICATION OF 316L STAINLESS STEEL (SS316L) FOAM VIA POWDER

COMPACTION METHOD

ZULAIKHA BTE ABDULLAH

A thesis as a partial fulfillment of the requirements for the award of the

Master of Mechanical Engineering

Faculty of Mechanical and Manufacturing Engineering

Universiti Tun Hussein Onn Malaysia

JUNE 2015

v

ABSTRACT

Metal foam is the cellular structures that made from metal and have pores in their

structures. Metal foam also known as the porous metals, which express that the

structure has a large volume of porosities with the value of up to 0.98 or 0.99. Porous

316L stainless steel was fabricated by powder metallurgy route with the composition

of the SS316L metal powder as metallic material, polyethylene glycol (PEG) and

Carbamide as the space holder with the composition of 95, 90, 85, 80, and 75 of

weight percent (wt. %). The powders were mixed in a ball mill at 60 rpm for 10

minutes and the mixtures were put into the mold for the pressing. The samples were

uniaxially pressed at 3 tons and heat treated by using box furnace at different

sintering temperature which are 870°C, 920°C, and 970°C separately. The suitable

sintering temperature was obtained from the Thermal Gravimetric Analysis (TGA).

There are several tests that have been conducted in order to characterize the physical

properties of metal foam such as density and porosity testing, and the morphological

testing (Scanning Electron Microscopy (SEM)), and Energy Dispersive X-ray

(EDX). From the result, it can be conclude that, the sintering temperature of 920°C

was compatible temperature in order to produce the metal foams which have large

pores. Other than that, the composition of 85 and 75 wt. % is the best compositions in

order to creates the homogenous mixture and allow the formation of large pore

uniformly compared to other compositions which in line with the objective to

produce foams with low density and high porosity which suitable for implant

applications. The average pore size was within range 38.555µm to 54.498 µm which

can be classified as micro pores.

vi

ABSTRAK

Logam berbusa adalah struktur sel yang diperbuat daripada logam dan mengandungi

liang-liang di dalam strukturnya. Logam berbusa yang juga dikenali sebagai logam

berliang yang mempunyai sejumlah besar keporosan pada strukturnya dengan nilai

sehingga 0.98 atau 0.99. Struktur berliang 316L keluli tahan karat telah difabrikasi

dengan kaedah metalurgi serbuk dengan komposisi serbuk logam SS316L sebagai

bahan logam, Polyethylene Glycol (PEG) sebagai penguat dan Baja Urea sebagai

pemegang ruang dengan komposisi bahan 95, 90, 85, 80, dan 75 peratus berat (wt.

%). Kesemua serbuk dicampur dan diadun menggunakan pengisar bebola pada

kelajuan 60 rpm dalam masa 10 minit dan campuran dimasukkan ke acuan untuk ke

proses penekanan. Sampel dipadatkan dengan tekanan 3 tan dan di sinter

menggunakan relau kotak pada suhu yang berbeza iaitu 870°C, 920°C, dan 970°C

secara berasingan. Suhu sinter yang sesuai diperolehi daripada Analisis Gravimetrik

Haba (TGA). Terdapat beberapa ujikaji dijalankan untuk mencirikan sifat fizikal

logam berbusa seperti ujikaji ketumpatan dan keliangan, ujikaji morfologi dengan

menggunakan Pengimbas Mikroskopi Elektron (SEM) dan Tenaga Serakan Sinar-X.

Daripada keputusan, dapat dirumuskan suhu sintering 920°C adalah paling sesuai

untuk digunakan untuk menghasilkan sampel dengan saiz liang yang besar. Selain itu

komposisi 85 dan 75 wt. % adalah komposisi terbaik, di mana komposisi ini

menghasilkan campuran yang homogen yang membenarkan pembentukan liang yang

besar secara seragam di mana selaras dengan objektif asal kajian untuk menghasilkan

logam berbusa yang mempunyai ketumpatan yang rendah dan keliangan yang tinggi

di mana sesuai untuk aplikasi implan. Purata saiz liang adalah dalam julat 38.555µm

hingga 54.498µm dan boleh diklasifikasikan sebagai liang mikro.

vii

TABLE OF CONTENT

Page

TITLE i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii-ix

LIST OF FIGURES x-xii

LIST OF TABLES xiii

CHAPTER 1 INTRODUCTION

1.1 Background of Study 1

1.2 Problem Statement 3

1.3 Objective of Study 3

1.4 Scopes of Study 3

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction 4

2.1.1 Metal Foams 5

viii

2.2 Metallic Material 6

2.2.1 Stainless Steel 8

2.2.2 Designation of Stainless Steel 11

2.2.3 Classification of Stainless Steel 13

2.3 Binder 15

2.3.1 Polyethylene Glycol (PEG) 16

2.4 Space Holder 18

2.4.1 Carbamide (Urea) 19

2.5 Fabrication Techniques 21

2.5.1 Metal Foam Fabrication Techniques 21

2.5.2 Fabrication Foams using Slurry Foaming Techniques 23

2.5.3 Fabrication Foams using Powder Metallurgy 25

Techniques

2.5.3.1 Powder Mixing 27

2.5.3.2 Compaction Process 28

2.5.3.3 Sintering Process 30

2.6 Research Parameters 32

2.6.1 Effect of Composition 32

2.6.2 Effect of Sintering Temperature 33

ix

CHAPTER 3 METHODOLOGY

3.1 Introduction 34

3.1.1 Flow Chart 35

3.2 Main Material 36

3.2.1 Raw Material 36

3.2.2 Polyethylene Glycol (PEG) 38

3.2.3 Carbamide (Urea) 39

3.3 Meshing Process of Space Holder Material 40

3.3.1 Crushing Process 40

3.3.2 Sieving Process 41

3.4 Powder Metallurgy Techniques 42

3.4.1 Mixing Process 42

3.4.2 Compaction Process 43

3.4.3 Sintering Process 45

3.5 Testing Method 48

3.5.1 Thermal Gravimetric Analysis (TGA) 48

3.5.2 Density and Porosity Test 49

3.5.3 Scanning Electron Microscopy (SEM) 51

3.5.4 Energy Diffraction X-ray (EDX) 52

x

CHAPTER 4 RESULT AND DISCUSSION

4.1 Introduction 54

4.2 Thermal Gravimetric Analysis (TGA) 54

4.2.1 Thermal Gravimetric Analysis 55

for Stainless Steel (SS316L)

4.2.2 Thermal Gravimetric Analysis 56

(TGA) for Carbamide (Urea)

4.2.3 Analysis of Thermal Gravimetric Analysis 58

(TGA) for polyethylene glycol (PEG)

4.3 Analysis of Density and Porosity Test 59

4.4 Scanning Electron Microscopy Analysis (SEM) 63

4.5 Energy Diffraction X-ray Analysis (EDX) 77

CHAPTER 5 CONCLUSION AND RECOMMENDATION

5.1 Conclusion 82

5.2 Recommendation 84

REFERENCES 86

APPENDIXES

xiii

LIST OF TABLES

Tables Page

2.1 Standard composition of 316L Stainless Steel 10

2.2 The sintering temperature and time of sintering for different metal 31

Powders

3.1 Material designation in accordance with Metal Powder Industries 37

Federation (MPIF) standard 35

3.2 Composition of SS316L Pure, SS316L with PEG and Carbamide 42

3.3 Labeling of Samples SS316L Pure, SS316L with PEG and Carbamide 43

3.4 Summary of maximum service temperature on Air for Stainless Steel 47

4.1 Result for density test of SS316L foams at sintering temperature of 59

870°C, 920°C, and 970°C

4.2 The average result for porosity test of SS316L foams at 61

sintering temperatures of 870°C, 920°C, and 970°C

4.3 Average of Pore Size for the SS316L Foams with Different Composition 71

4.4 Element Mass Percentage Presence at Samples for EDX Analysis at 77

Sintering Temperature of 870°C

4.5 Element Mass Percentage Presence at Samples for EDX Analysis at 78

Sintering Temperature of 920°C

4.6 Element Mass Percentage Presence at Samples for EDX Analysis at 79

Sintering Temperature of 970°C

4.7 Element mass percentage presence at Samples for EDX analyses of 80

the structure that represent the corrosion are on the sample with

composition of 75 wt. % at sintering temperature of 870°C

1

CHAPTER 1

INTRODUCTION

1.1 Background of Study

At present, high requirement of lightweight constituent make metal foams extremely

attractive as an industrial technology for biomedical application which is demanding

on weight reduction (Gauthier, 2008). The physicality of metal foam is high porosity

make metal foams very lightweight. Basically, metal foams are artificial porous

medium that has solid matrix structure of metal consist of empty or fluid-filled voids.

Metal foams have been characterized into two types which are open-cell and closed-

cell. The foams is called open-cell when the voids are connected via open pores, but

when the foams are separated by the solid walls and not connected via open channel

are described as closed-cell (Dukhan, 2013).

In order to achieve the metal foams which suitable in orthopedic application

and have good properties such as low density, high strength-to-weight ratio, excellent

mechanical properties, biocompatibility and corrosion resistance, the suitable

materials have to choose carefully. Metals are the most suitable material to fabricate

the metals foam proportionate to the ceramic and polymer. Even though the ceramic

material have excellent corrosion resistance but ceramic cannot being employed as

load bearing implants due to their brittle properties, whereas polymeric systems

cannot sustain the mechanical forces present in joint replacement surgery (Ryan,

Pandit & Apatsidis, 2006). There are various types of metal that have been used as

main materials to fabricated metal foams includes titanium, titanium alloys, nickel,

aluminum, magnesium, and stainless steel(Rosip et al., 2013).

2

Since early 1960s, Stainless steel widely used in orthopedic application such as

fabrication of femoral stems, balls and acetabular cups, fabrication of knee and

femoral components and tibial trays because of its biocompatibility and inexpensive

(Davis, 2003). Porous Stainless steel is compatible to use as a coating on Stainless

steel implants. The methods that use to fabricate these coating are by sintering beads

or thermal spray techniques. The oxides that formed on the surfaces of stainless steel

is more stable than the oxides formed if using titanium and titanium alloys and leads

to the crevice corrosion and degradation of the implants. Because of that, Stainless

steel is choosing to replace other materials over the year. Stainless steel has been

approved in terms of mechanical properties and clinical trials by the US Food and

Drug Administration (FDA). Its mechanical properties are often used as bench mark

criteria to evaluate other alloys for stent applications (Chen et al., 2014).

There are large varieties of fabrication techniques for metal foams or similar

porous structures but usually favorable technique is liquid phase or powder

metallurgy process. By using compaction method, metal powders are mixed with

foaming agent and then compacted by using hot pressing, cold pressing, hot

extrusion, or co-extrusion. The final product of the compaction process is a dense

foamable material that can be worked into sheets and profiles (Banhart &

Baumeister, 1998) Slurry method are commonly used to produce metal foams by

providing metal powder, blowing agent and a binder that mix together and the

mixture poured into mould and left to the elevated temperature until melting

temperature(Rosip et al., 2013). Casting process also has been used to produce metal

foams around inorganic hollow spheres with a liquid melt or using open porosity

polymer foams as starting points.

This research is to fabricate the 316L Stainless Steel (SS316L) foam prepared

by Compaction technique and to study and characterize the properties of SS316L

foam after sintering process. The SS316L have used as a raw material and

Polyethylene glycol (PEG) and Carbamide are used as a binder and space holder

respectively. The material will be mixed by using ball milling machine to get the

homogenous mixture. After that the compaction process will be held by using

conventional axial pressing. This process is known as powder metallurgy technique.

The Properties Characterization will be measured by doing density and porosity test,

Thermal Gravimetric Analysis (TGA), Scanning Electron Microscopy (SEM), and

Energy Diffraction X-ray (EDX).

3

1.2 Problem Statement

The major challenges that need to focused while producing metal foam is the

mismatch of the properties between bones and the metallic material. Due to this

mechanical mismatch, bone is insufficiently loaded and become stress shielded,

which eventually leads to bone resorption (Ryan et al., 2006).

Thus, there are factors need to be considered includes the interconnecting

pores that suitable with bone, the pores of the implants same with the pores of bone,

the shape and the density of implants is same with the shape and density of bones.

1.3 Objectives

The objectives of this research are:

i) To fabricate the 316L Stainless Steel (SS316L) as metallic cell

prepared by Compaction technique.

ii) To characterize the properties of fabricated SS316L foam after

sintering process.

1.4 Scope of Study

The scope of this research includes:

i) The percentage of the composition for the SS316L powder are 95, 90,

85, 80 and 75 of weight percent (wt. %) respectively, for the

Polyethylene glycol is 1 of weight percent (wt. %) and balance is for

the Carbamide composition.

ii) The sintering temperatures that have been choosing are 870ᴼC, 920ᴼC,

and 970ᴼC.

iii) The characterization for the properties of metal foam will be study by

conducted the Thermal Gravimetric Analysis (TGA), Density and

porosity test, Scanning Electron Microscopy (SEM), and Energy

Diffraction X-ray (EDX).

4

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

This chapter justify about literature review of the research to gather information and

knowledge of the material that need to be used in this research, the method that have

been used in previously study and the method that has been select in this research.

This section focused on the Stainless Steels and its properties, the binder and space

holder that has been used and the fabrication method that involved.

5

2.1.1 Metal Foam

Basically, metal foam can be described as the cellular structure that made from metal

and have pores in their structures. Metal foam also known as the porous metals,

which express that the structure has a large volume of porosities with the value of up

to 0.98 or 0.99. The high porosity contributes lightweight to the metal foams.

Besides, the terms foamed metal or metallic foam illustrate the fabrication or forming

process of the metal foam.

Metal foams have been characterized into two types of structure which are

open cell and closed cell. The structure of the porous metals influences the

applications of the metal foams. Closed cell foam can be described as the pores is fill

with gas and separated from each other by metal cell walls, have good strength and

usually used in structural application. Open cell foam contain a continuous network

of metallic struts and the enclosed pores in each strut frame are connected, the

strength are weaker than the closed cell and are mainly used in functional

applications where the continuous nature of the porosity is exploited (Kennedy,

2012). Figure 2.1 show the example of the microstructure of closed cell foam and

open cell foam for metal foams.

Figure 2.1: Microstucture of (left) a closed cell foam and (right) an open cell

foam (Kennedy, 2012)

6

There are other application of metal foams such as energy absorber, heat

exchangers, mechanical damping and in filters system. Nowadays, metal foams has

been use in biomedical application as bone implants. Metal foams have excellent

potential for implant application due to its low density and good combination of

properties because of the reduced stiffness mismatches. Other than that, it is

important to make sure the bone ingrowth which possible by metal foams and greatly

improve the bone implant interface and may allow for efficient soft tissue attachment

supplementing the stability of the implant by biological fixatation (Mariotto et al.,

2011).

2.2 Metallic Material

The excellent of electrical and thermal conductivity and mechanical properties make

metal used as biomaterials which suitable used as biomedical materials. Biomaterial

can be expressed as any material used to make devices to replace a part or a function

of body in a safe, reliable, economic, and physiologically acceptable manner (Park &

Lakes, 2007). Metals offer excellent strength and resistance to fracture, which

suitable for the medical application that requiring load bearing.

There are number of metallic material that good biocompatibility which are

not cause serious toxic reaction in human body such as Stainless Steels, Cobalt

Alloys, Titanium Alloys and noble metals. These metals are suitable for the

structural application in the body such for implants for hip, knee, ankle, shoulder,

wrist, finger, or toe joints. These metals have the ability to bear significant loads,

withstands fatigue loading, and undergo plastic deformation prior to failure, made

these metals are popular chosen as material for the implant application (Shi, 2006).

Some metals have high corrosion resistances which made its suitable used as

passive substitutes for fracture healing aids as bone plates and screws, spinal fixation

devices, and dental implants. Corrosion can be defined as the unwanted chemical

reaction of a metal with its environment, resulting in its continued degradation to

oxides, hydroxides, or other compound. It is important to choose metallic materials

which have high corrosion resistance due to its biocompatibility in human body.

7

Other than that, metallic material play active roles in devices such as vascular stents,

catheter guides wires, orthodontic arch wires, and cochlea implants (Park & Lakes,

2007). Figure 2.2 show the typical implant applications for orthopedic purposes.

During the twentieth century, the first metallic material was successfully used

in orthopedic applications were Stainless Steels and Cobalt-Chrome Based Alloys

(Navarro et al., 2008). The Stainless Steels and Cobalt-Chrome Based Alloys have

high corrosion resistance based on the presence of Chromium in their properties

make it has the ability to render the alloys passive. In this research, the metallic

material that has been used is Stainless Steels which will be explained later.

Figure 2.2: The typical implant applications for orthopedic purposes (Davis,

2003)

8

2.2.1 Stainless Steels

Stainless Steels are widely used in modern metallurgy. Historically, Stainless Steel

was discovered in early 18th and 19th centuries; when the identification of Chromium

as an element was begin. Harry Brearley (1871-1940) who first produced the first

commercial cast of martensitic Stainless Steel, which used for cutlery area (Cobb,

2010).

Different from other alloy systems that has widely used such as Carbon

Steels, Alloy Steels, and Aluminum Alloys, that are relatively dilute solution of

several element in the parent matrix. Stainless Steel is one of alloys system that it is

not a dilute solution. There are several percent of alloying element that contained in

Alloys steels includes Carbon, Manganese, Nickel, Molybdenum, Chromium, and

Silicon, Impurities Sulfur, Oxygen, And Phosphorus. Typically, Alloy Steels contain

very small of amounts of Titanium, Niobium and Aluminum. In contrast, in about

11% of Chromium that contain alone in Stainless Steel (McGuire, 2008). Usually,

the shape of Stainless Steels particles is irregular. Figure 2.3 show the basic shape of

metal powder (Kalpakjian & Schimid, 2006).

Figure 2.3: Basic shape of Metal powder (Kalpakjian & Schimid, 2006)

9

Basically, Stainless Steel is Iron-Base Alloys that contain a minimum of

approximately 11% of Chromium (Cr), amount that needed to prevent the formation

of rust in unpolluted atmosphere. There are few Stainless Steels contain more than

30% Cr or less than 50% Ferum (Fe). They fulfill the Stainless Steel characteristics

condition by the formation of an invisible and adherent Chromium rich oxide surface

film. The process is happen when the oxide forms and heals itself in the presence of

oxygen (Davis, 1994).

There is development of Stainless Steels that used in implant application. The

first Stainless Steel that has been recorded that utilize as implant fabrication was the

18-8 type which is stronger and more resistance to corrosion than the Vanadium

Steel. There are some improvement has been done to Stainless Steel by attached a

small percentage of Molybdenum to improve the corrosion resistance in Chloride

solution which next the Stainless Steel has known as type 316 Stainless Steels.

During 1950s, they upgrade the Stainless Steel by reducing the amount of Carbon

content from 0.08 to a maximum amount of 0.03% to increase corrosion resistance

and minimize the sensitization and become as type 316L Stainless Steels. The

Chromium content also function as the reduction of Carbon content in which to

impart corrosion resistance in Stainless Steel which the concentration of Chromium

is 11% (Wong & Bronzino, 2007).

The type of Stainless Steels that most widely used for implant fabrication is

the austenitic Stainless Steels which is type 316 and 316L. These groups of steel are

nonmagnetic and have better corrosion resistance than others because the content of

Molybdenum that enhances the resistance while attach in the salt water. The

American Society of Testing and Material (ASTM) recommend type 316L rather

than type 316for fabrication for implant. The standard composition of 316L Stainless

Steel that follows American Society for Testing and Materials is show in Table 2.1

(Wong & Bronzino, 2007).

10

Table 2.1: Standard composition of 316L Stainless Steel (Wong & Bronzino,

2007)

Element Composition (%)

Carbon 0.03 max

Manganese 2.00 max

Phosphorus 0.03 max

Sulfur 0.03 max

Silicon 0.75 max

Chromium 17.00-20.00

Nickel 12.00-14.00

Molybdenum 2.00-4.00

Ferum Balance

Mariotto et al., (2011) in the study of fabrication metal foam for biomedical

application have used Stainless Steel type 316L as main material. Ammonium

Carbonate and Ammonium Bicarbonate is chosen as foaming agents that will mix

with the Steel powder. The technique that has been applied to fabricate the metal

foam is by using powder metallurgy techniques and the metal foam vacuum heat

treated in 2 stages, the first stage at 200°C for 5 hours to decompose the carbonate

and the second stage at 1150°C for 2 hours to sinter the steel. From the research, the

density of the metal foam is in about 0.3 g.cm? ? and 0.5g.cm? ? for Ammonium

Carbonate and Ammonium Bicarbonate foaming agents respectively. The

microstructure of the metal foam is heterogeneous pore structures, composed by

irregular and isolated pores. The result show that the metal foam produced can be

viable in orthopedic implants (Mariotto et al., 2011).

The previous researchers produce of 316L Stainless Steels foam via slurry

method. In the research, the 316L Stainless Steel is chosen as metallic material,

where the Polyethylene Glycol (PEG) and Methylcellulose (CMC) were used as

binder. The result shown that, the SS316L metal foams had been successfully

produced by using slurry method. The microstructure observation show that the

metal foams had closed pore than open pore and the powder particles did not well

growth to combine each other after sintering process. At the end of the research, the

suitable composition of SS316L for metal foams fabrication is gained with the

suitable sintering temperature which resulted better strength of metal foams (Rosip et

11

al., 2013). Gauthier (2008) in his study of structure and properties of open-cell 316L

Stainless Steels foams produced by a powder metallurgy-based process used type

316L Stainless Steel as main material, a polymeric binder, and chemical foaming

agent. All the material was dry mixed, then the mixture was molded and heat

treatment is applied to the metal foam. From the result of the investigation, the 316L

Stainless Steels foams was produced and the heterogeneous foams feature a mix of

open and closed pores depending on processing conditions. The mechanical

properties that get from the test show that the value is following the scaling for metal

foams and was related to foam density (Gauthier, 2008).

In this research, the 316L Stainless Steels is choosing as main material. The

characteristic of the 316L Stainless Steel will be elaborate later in the next topic.

2.2.2 Designation of Stainless Steels

The designation of Stainless Steels can be divided into three different designation

systems in principle which are the designation of wrought Stainless Steels, cast

Stainless Steels and P/M Stainless Steels. Basically, in United States the American

Iron and Steel Institute (AISI) numbering system and the Unified Numbering System

(UNS), are design the wrought grades of Stainless Steels. Other major industrial have

also been established other designation system of Stainless Steels but it also similar

to the AISI. Basically for wrought Stainless Steels, the UNS designation consist of

the letter “S”, follow by a five-digit number whereas for those alloys that have an

AISI designation, the first three digit of the UNS designation usually correspond to

an AISI number. The UNS designation consists of the letter “N” followed by a five-

digit number when Stainless Steels that contain high Nickel content (>25 to 35%)

(Reardon, 2011).

There are institutions which call as High Alloy Product Group of the Steel

Founder’s Society of America, also known as the Alloy Castings Institute (ACI), has

designated standard cast Stainless Steels grades. The composition and properties of

the Stainless Steels similar to the wrought grades, and some cast Stainless Steels are

modified in order to improved cast ability and the higher silicon levels are typically

12

used in cast Stainless Steels for the improvement. The designated of Cast Stainless

Steels as alloys intended primarily for liquid corrosion service (C) or high-

temperature service (H). The nominal Chromium-Nickel type of the alloy denotes by

the second letter of the designation and it will change when the Nickel content

increases. The maximum Carbon content (percentage x100) is represented by the

numeral or numerals following the first two letters. When there are another alloying

element is present, these are indicated by the addition of one or more letter as a

suffix. So that, from the Figure 2.4 it can be described that the designation of CF-8M

refers to an alloy for corrosion-resistant service (C) of the 19Cr-9Ni type, with the

maximum Carbon content 0.08% and containing Molybdenum (M) (Davis, 2000).

Figure 2.4: Chromium and Nickel contents in ACI standard grades of

heat-resistant and corrosion-resistant Stainless Steel castings (Davis, 2000)

The Metal Powder Industries Federation (MPIF) establishing the designation

for P/M products. The popular grades of wrought Stainless Steels have been taking

as mark of derivation of new composition of P/M Stainless Steels. Because of that,

the characteristics of most P/M Stainless Steels are parallel to the wrought Stainless

Steels. The wrought Stainless Steels have a maximum of Carbon content of 0.08% or

higher but there are few selected of wrought Stainless Steels are available in a low

Carbon with maximum Carbon content of 0.03% and these are designated as “L”

grades. For the P/M Stainless Steels, with the exception of the martensitic grades, are

specified to be “L” grades or the low Carbon of the alloys. There are reasons on why

the P/M Stainless Steels need for the low Carbon requirement includes low Carbon

13

content renders the Stainless Steels powder soft and ductile, making it easier to

compact, and it minimize the potential for Chromium Carbide formation or being

sensitive during cooling from the sintering temperature (Davis, 2000).

2.2.3 Classification of Stainless Steels

The classification of Stainless Steels can be divided into five which are

martensitic, ferritic, duplex, and austenitic which based on the characteristic

crystallographic structure of the alloys while the fifth is the precipitation-hardenable

alloys which based on the type of heat treatment used. Figure 2.5 illustrate the

common crystal structure of metal such as body-centered cubic (BCC), face-centered

cubic (FCC), and hexagonal close-packed (HCP) (Shi, 2006).

Figure 2.5: The common crystal structure of Metal such as body-centered cubic

(BCC), face-centered cubic (FCC), and hexagonal close-packed (HCP) (Shi, 2006)

Martensitic Stainless Steel can be described as Fe-Cr-C alloys that possess

body-centered tetragonal crystal structure (martensitic) in the hardened condition.

The properties of martensitic Stainless Steel are ferromagnetic, hardenable by heat

treatments, and generally resistant to corrosion in mild environments. These types of

Stainless Steel have high hardness value make it suitable for dental and surgical

instruments. Basically, ferritic Stainless Steel contain Iron-Chromium Alloys with

14

body-centered cubic (bcc) crystal structures. The applications of these types of

Stainless Steel are solid handles for instruments, guide pins and fastener. Duplex

Stainless Steel is a two phase alloys which have equal proportions of ferrite and

austenite phases in their microstructure and characterize by their low carbon content,

additions of molybdenum, nitrogen, tungsten, and copper. Different from other types

of Stainless Steel that have applications in biomedical, duplex Stainless Steel are

commonly used in the oil and gas, petrochemical, pulp and paper industries. The

precipitation-hardenable (PH) Stainless Steel is the only type of Stainless Steel that

classified by the heat treatment. Basically, these types of Stainless Steel are

Chromium-Nickel grades that can be hardened by an aging treatment which can be

classified into three categories which are austenitic, semi-austenitic and martensitic.

Usually, the precipitation-hardenable (PH) Stainless Steel has applied in medical

such as neurosurgical aneurysm and micro vascular clips and various types of

surgical and dental instruments (Davis, 2003).

In this research, the type of the Stainless Steels that will be used can be

classified as austenitic Stainless steels. Austenitic Stainless steel has largest numbers

of alloys and use. Same with the ferritic Stainless steel, the austenitic Stainless Steel

cannot be hardened by heat treatment. The other properties for the austenitic

Stainless steels are nonmagnetic in the annealed condition and can be hardened only

by the cold working. The application for the austenitic Stainless Steel such dental

impression trays, guide pins, holloware, and hypodermic needles, sterilizer, and other

application that have a complex shape. These type of Stainless Steels also has widely

used for implants (Davis, 2003).

15

2.3 Binder

Binders play an important role in processing of component by using pressing

method. Basically binders can be described as multi-component mixtures of several

Polymers which consist of some additives that add to the primary component include

dispersants, stabilizers, and plasticizers. The function of the binders is to assist in

shaping of the component during the process of fabrication to provide strength to the

shaped component. Binders also hold the metal particles together until the onset of

sintering. The density of a pressing (small pressed part) increases with the use of a

binder plasticizer). The binder’s properties can influence on the metal particles

distribution, shaping process, dimensions of the shaped component, and the final

properties of the sintered component. The binder provides strength which can

avoided the green body to the formation of defects such as cracking and blistering

(Heaney, 2012). The binder will be eliminated from the shaped component during

the firing process without any disruptive effect (Angelo & Subramanian, 2008).

There are ideal characteristics that binder should have followed. A good

binder that used in shaping must have low inherent viscosity at the shaping

temperature, must be strong and rigid after shaping, and should have low molecular

weight. When the binder regard to powder interaction, a binder must wet and adhere

to the powder, be chemically passive, and be stable during mixing and shaping. After

shaping process, the shaped component will be firing and the binders will be burnout.

So that, a binder must be noncorrosive and giving nontoxic products on

decomposition leave no residue on burnout, have a decomposition temperature above

and below mixing temperature. From the context of manufacturing process, a binder

that used for shaping must be readily, safe and low cost, have long shelf-life, low

water absorption and no volatile components and have high strength and stiffness

(Angelo & Subramanian, 2008).

There are investigations of producing the metal foams which contain binder.

The previous researchers study the production of micro-porous austenitic Stainless

Steels by powder injection moulding route. The material that have been used in order

to produce porous structures is 316L Stainless Steel as the main material, a multiple-

component binder system consist of Paraffin wax (PW), Polypropylene (PP),

16

Carnauba wax (CW), and Stearic Acid (SA), and spherical Poly(Methyl

Methacrylate)(PMMA) particles were used as a space holder material (Gülsoy &

German, 2008). Other investigation that produce the metallis foams was the study of

fabricating 17-4 PH Stainless Steels foam for biomedical application used

Polyvinylalcohol (PVA) as binder which added to the mixture of Stainless Steel and

Carbamide to increase the green strength of the sample (Mutlu & Oktay, 2013). Open

cell Fe-10% Al Foam was fabricated by using space holder technique by mixing

Ferum and Aluminum solution (FeAl) with Natrium Cloride (NaCl) and Polyester

resin as space holder and binder respectively in order to study the characterization of

the foams (Golabgir et al., 2014).

2.3.1 Polyethylene Glycol (PEG)

Polyethylene Glycol (PEG) is available in molecular weight from 400 to 12000 but

low molecular weights are liquids at room temperature. The most compatible

molecular weight of PEG is from 6000 to 12000 which are used in most application

at higher end. There are many type of PEG such as waxy solids that are soluble in

water, Methylene Chloride, and Acetone. PEG is clean chemical that burnout during

firing process. PEG has a low strength binder, and it is sufficient for most purpose.

Usually, PEG is added as solution or in fluid emulsion condition in order to obtain a

homogenous fluid without making a binder clotted (King, 2001). PEG is water-

soluble Polymer and frequently used as a binder, plasticizer or a lubricant. PEG

binder is about 10 times more pure compared with Polyvinyl Alcohol (PVA)

(Watchman, 2009).

There are many series of PEG members that can be categorized by its

molecular weight. The average molecular weights of 200 to 20,000 can be

categorized as the intermediate members which can be produced by the Sodium or

Potassium Hydroxide-Catalyzed batch polymerization of Ethylene Oxide onto water

or Monoor Diethylene Glycol. The fabrications of the Polymer are by stepwise

anionic addition polymerization and therefore possess a distribution of molecular

weights. Usually, the application of this type of PEG is commercial uses for products

17

in metal forming, Ceramic and Rubber –processing operations. The molecular

weights from 100,000 to 10,000,000 can be categorized as the highest members of

PEG. The fabrication of these members is by using the special anionic

polymerization catalysts that incorporate Metals such as Aluminum, Calcium, Zinc,

Iron, and coordinated ligands includes Amides, Nitriles, or Ethers. This type of PEG

is widely used because of their ability at very low concentration to reduce friction of

flowing water (Schwartz, 2002).

The application of PEG can be found in medical and biotechnical because of

the range of properties. In medical and biotechnical applications, the needs of

amphilicity behavior make PEG has widely used because of these Polymers of

Ethylene Oxide can be described as amphiphilic which can soluble both in water and

in most organic solvents. Other than that, PEG also can be found in Ceramic

applications which used as components of Ceramic slips and glazes for the

manufacture of Porcelain signs and other Vitreous coatings (Schwartz, 2002). Other

application of PEG is as a binder in manufacturing for shaping components such as

can be used in injection molding process, casting, extrusion, and powder compaction.

Figure 2.6 show the PEG that will be used in this study.

There are previous researchers that use PEG as binder such as the

investigation of fabrication of 316L Stainless Steels foam by using slurry method

used binder such PEG and Methylcellulose (CMC) (Rosip et al., 2013). There are

other researchers that used the same method of fabrication in order to investigate the

influence of composition and sintering temperature on density for pure and Titanium

Alloy foams. The Titanium slurry was prepared by mixing pure Titanium and

Titanium Alloys powder with PEG as a binder to fabricate the foams (Ahmad et al.,

2010). Rafter et al. in her study of development of SS316L foams with different

composition using compaction method use PEG as binder that mixed with SS316L

powder and crystalline Sugar as space holder (Rafter et al., 2014).

18

Figure 2.6: Polyethylene Glycol (PEG)

2.4 Space Holder

The terms Space holder play important role in order to produce metal foams. It is

important to make sure the suitable space holder is selected. The basic characteristics

of the space holder is has to be easily removable later by the thermal decomposition

either by rinsing it out or by leaching in solvents like water or alcohol. The good

criteria of space holder is its can be removed without leaving any residues and any

reaction does not occur with the metal powder, which can affect the metal powder if

the powder contains highly reactive elements like Titanium or Magnesium (Dukhan,

2013).

The size and shape of space holder also have to carefully select because of

the pores are connected to the space holder granules, elongated pores or pore

structure and tailored density gradient. When the selection process is finished, the

next process is the space holder will be mixed together with specific metal powder

and compacted into a desired shape component by conventional techniques such

axial pressing or isostatic pressing. The higher force that applied to the compact part

during compaction process due to get higher compaction pressure to make the part

have strength and the next process can be instituted. It is important to make sure the

19

strength of the compact part is sufficient enough to survive after the space holder will

be removed during sintering process (Dukhan, 2013).

There are many different space holder material that widely used includes

Polymeric material, Sodium Chloride, Crystalline Sugar Cane, Sodium Fluoride,

Carbamide, Ammonium Hydrogen Carbonate, Magnesium and Tapioca Starch

(Rafter et al., 2014). In this study, the space holder that used is Carbamide or also

known as Urea. Basically, the shape of the Carbamide is spherical and it is high

solubility in water.

Previous researcher has applied the space holder in the fabrication of metal

foams. The research to investigate the compressibility of porous Magnesium foams

which fabricated via powder metallurgy method and Carbamide (CO (NH2)2) is

choose as space holder particles (Wen et al., 2001). Other than that, other researchers

study the fabrication porous 316L Stainless Steels with two different space holder

which are Ammonium Carbonate and Ammonium Bicarbonate to compare the

properties of the metal foams by using powder metallurgy techniques (Mariotto et

al., 2011). The pure Titanium and Titanium Alloys foams were fabricated by using

the slurry method to produce the metal foams with addition the Polyethylene Glycol

as the binder and the Methylcellulose as the space holder. This aim of this

investigation was to study the characterization of the pure Titanium and Titanium

Alloys foams (Ahmad et al., 2014). The other research that involved binders

consumption is the study of powder metallurgy synthesis of Steel foams has use the

Steel powder which contain of 2.5% Ferum (Fe) and 0.2% Carbon (C) which are the

two materials that had been choose as space holder which are Magnesium Carbonate

(MgCO3) and Strontium Carbonate (SrCO3) (Park & Lakes, 2007).

2.4.1 Carbamide (Urea)

The types of space holder that frequently used are Carbamide and Ammonium

Hydrogen Carbonate. Carbamide can be removed by treatments at high temperature

either before sintering or as an initial step in the sintering heat treatment. Carbamide

is used in metal foam fabrication in order to produce porosity for certain higher

melting point metals. Carbamide also has been used as space holder for Aluminum

20

and Stainless Steel but it not being removed thermally but by water leaching. Figure

2.7 show the Carbamide particles. The particles of Carbamide have different shapes

such as rough spheres or high aspect ratio flakes. The varieties of shape for

Carbamide allow the pore shape to be controlled as this shape is preserved in the

porous material (Chang & Zhao, 2013).

Figure 2.7: Carbamide particles

There are previous researchers that used Carbamide as the space holder

material in fabrication of metallic foam. The high porosity Titanium foam was

fabricated by using Carbamide as the space holder material by applying space holder

techniques was study by the Kotan & Bor (2007). The Carbamide were dissolved in

water before mix with the Titanium powder and compacted by using conventional

pressing machine to produce Titanium foams (Kotan & Bor, 2007). The

compressibility of porous Magnesium foams which fabricated via powder metallurgy

method was studied and the Carbamide (CO (NH2)2) is choose as space holder

particles (Wen et al., 2001). The characterizing of the CR-Si-Ni-Mo Steel foams use

CR-Si-Ni-Mo Based Ancorsteel 4300 powders with the Polyvinyl Alcohol (PVA)

and Carbamide as binder and space holder material respectively was studied by

Mutlu and Oktay (2011).

21

2.5 Fabrication Techniques

There are various processes that can be classified according to the state of the

starting Metal to liquid, powdered, ionized. For liquid Metal, it can be foamed

directly by injecting gas, gas-releasing foaming agents or by producing

supersaturated Metal gas solutions. Other method that has been used such investment

casting and usage of filler materials. Metal foams also can be produced by using

Metal powder as starting materials which mix the powder with foaming agents and

compacted to a foamable precursor. The Metal slurries techniques also usually used

in order to produce Metal foams by using Polymer or powder mixtures (Banhart &

Baumeister, 1998).

2.5.1 Metal Foams Fabrication Techniques

The fabrication of Metallic foam was first start by Benjamin Sosnik in year 1948. He

produces Metallic foam by creating pores from apply Mercury vapor to molten

Aluminum and Mercury. At year 1956, the production of Metallic foam has been

done by replacing the Mercury with foaming agents generating gas by thermal

decomposition, while during year 1963 there was development of metallic foams by

a powder compacting techniques. After the sequence of history of Metallic foams

fabrication from the previous researcher, until now, many researcher have developed

Metallic foams using the techniques that avoid applying Mercury that caused the

toxicity (Chang & Zhao, 2013).

Basically, the fabrication of metal foams can be divided into two categories

which are foams made from metallic melts or in the liquid route and foams made

from solid metals or can be called as solid state route. In liquid route, it can be

described that the porosity can be derived in the form of gas bubbles either by

injecting the gas from outside or by inside by decomposed of some chemical such

like Titanium Hydride (TiH2). By contract, in solid state route, the metal powder

were used and mixed with other suitable powder or binder which can be removed

later by dissolution or decomposition such as firing process (Bhatnagar & Srivatsan,

22

2009). Figure 2.8 show the four categorizes of metal foams fabrication process

(Banhart, 2001).

Figure 2.8: Overview of the various fabrication processes of Metal foams (Banhart,

2001)

There are various fabrication method that involved in liquid sate route

categorize. The basic of the fabrication of metal foams is by direct foaming of melts.

That is impossible to directly injecting gas into the liquid in order to produce metallic

melts of foams. This is because, the high density of the liquid, the buoyancy forces

are high, and the gas bubbles that formed to the metallic melt will be influence to the

collapse of the foams. This problem can be solved by increase the viscosity of the

molten metal, modifying the surface energy of the liquid limit and etc. Investment

casting is also the method that can be categorized in liquid state route. In investment

casting, a thermo-physical shock treatment also known as reticulation is applied to

the polymeric foam. Basically, the material that used as start material is Polyurethane

foam (Dukhan, 2013).

The Syntactic foams can be produced by melt filtration which these foams

consist of a metallic matrix that attaches light weight elements. Usually, hollow

spheres are used which can be made of Glass, Ceramic, Fly Ash or Metal and

includes homogenous on it and generated closed pore structure. High pressure die

23

casting process also the method for the production of foams or sponges by comprises

the infiltration of Aluminum, Magnesium, Or Zinc melts into Polymer structures.

There are three types of fabricated metal foams by melting of foamable precursors

which are precursors produced by powder metallurgical methods, precursor

processing in the semi-solid state, and precursor fabrication from the liquid state

(Dukhan, 2013).

Nowadays, there is fabrication that no liquid metal involved, in other words

the fabrication of metal foams via solid state route. The first method to produce open

porous foams is using space holders. The suitable space holder must be select to

make sure this space holder will be easily removed later by thermal decomposition.

Next, the slurry foaming techniques is widely used in order to produce metal foams.

This method involves metal powders and a suitable carrier fluid which can be

described as solvent and polymeric binder. The syntactic foams that made via

powder metallurgy techniques are required for materials with a high melting point

like steels which melt infiltration cannot be carried (Dukhan, 2013).

In this study, the fabrication of metal foams that involved is by solid state

route. The metal foams will be produce via powder metallurgy techniques which

involved sintering and dissolution process in order to synthesize Stainless Steel metal

foams and physical properties of the foam after sintering process.

2.5.2 Fabrication Foams using Slurry Foaming Techniques

Slurry foaming technique can be described by preparing slurry which the contents is

involved, metal powders, blowing agents and some reactive additives. The next

process is to pour the slurry into mould after mixing process and left at elevated

temperatures. The additive and the blowing agent influence the slurry turns viscous

and starts to expand as gas. When the slurry can be considered as in stable condition,

the slurry need to dry completely and the sintering process will be begin. The

sintering process done for produce final product of metal foams with ideal strength.

24

Usually, slurries foaming techniques has be done in order to produce open

porous metal foams. As the example, the open porous polymer foam is dipped into a

slurry that containing a mixture of Silver and Silver Oxide powder. The foams is

coated and heated up to certain temperature, and make the Polymer inside the slurry

will be burn out and densify the metal powder particles to produced net-shaped parts

and start the sintering process and forming a rigid cellular metal structure.

There are previous researchers study the fabrication of metallic foam via

slurry method route obtains the result of density of foams down to 7% have been

achieved but there are problems with low strength and cracks in the foamed material

(Banhart, 1971). Zhou et al. in his research to fabricate Stainless Steels foam was use

slurry method to produce the metal foams with the additive of Polyurethane sponge

and high-purity Argon to the Stainless Steel powder (Zhou et al., 2008). The

morphology analysis of the Stainless Steels foams is show in Figure 2.9.

Figure 2.9: Scanning Electron Morphology (SEM) analysis of Stainless Steels

foams via Slurry Method (Zhou et al., 2008)

Ahmad et al. proposed that slurry foaming techniques in order to produce

Titanium foam with addition of PEG and CMC as binder and space holder

respectively. Polyurethane (PU) has been used as a scaffold. In the study also has

mention that slurry foaming technique can produce open celled and high porosity of

Titanium foam (Ahmad et al., 2010).

86

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